1. Field of the Invention
The present invention relates generally to a lock-up controlling apparatus for an automatic transmission. More particularly, the present invention relates to improvement of a lock-up controlling apparatus of the foregoing type.
2. Description of the Related Art
Conventional typical lock-up controlling apparatuses each employable for an automatic transmission are disclosed in Japanese Patent Application Laying-Open Nos. 61-116170 and 2-203072.
To facilitate understanding of the present invention, each of the conventional lock-up controlling apparatuses as mentioned above will briefly be described below.
The lock-up controlling apparatus disclosed in Japanese Patent Application Laying-Open No. 61-116170 includes as essential components a fluid coupling interposed between the output shaft of an engine and the input shaft of a transmission for transmitting a torque outputted from the engine to the input shaft of the transmission, a direct connection clutch disposed in parallel relationship relative to the fluid coupling for operatively connecting the output shaft of the engine directly to the input shaft of the transmission in preference to the fluid coupling as desired, a load sensor for detecting loaded states of the engine, a vehicle speed sensor for detecting the running speed of a vehicle, and a shifting mechanism for shifting the direct connection clutch to the operative state when it is found that load detected by the load sensor and vehicle speed detected by the speed sensor lie within a predetermined direct connection range defined by a desired engine load range and a desired vehicle speed range. In addition, the lock-up controlling apparatus includes a shifting operation postponing unit which is disposed in the shifting mechanism for gradually reducing an intensity of operative connecting force of the direct connection clutch in order to prevent the torque appearing on the output shaft of the transmission from being quickly reduced when the shifting mechanism is shifted from the direct connection range to the non-direct connection range having a high magnitude of load.
On the other hand, the lock-up controlling apparatus disclosed in Japanese Patent Application Laying-Open No. 2-203072 includes a lock-up clutch mechanism for controllably setting an intensity of an operative connecting force to a predetermined value within a predetermined working range determined on the basis of engine load and the number of revolutions of a power transmission system. When a transition to a working range where a lock-up clutch is completely released is detected within a predetermined working range where the lock-up clutch is held in the half operatively connected state or the like, the operative connecting force of the lock-up clutch is controlled in such a direction as to allow an intensity of the same to be reduced from the point of time within the predetermined working range.
However, with each of the conventional lock-up controlling apparatuses each employable for an automatic transmission discussed above since an intensity of operative connecting force of the lock-up clutch is gradually reduced regardless of the present state of the engine load when the lock-up clutch is shifted to the released state, there arises a malfunction that the releasing of the lock-up clutch from the operative state is delayed at the time of coasting of a vehicle. As a result, since an engine brake functions in excess of a predetermined level, there appears a problem that a diver unpleasantly feels a stifling shock in the course of coasting of his vehicle. Also in the case that a throttle valve is largely opened to assume a large throttle valve opening degree, since an intensity of operative connecting force of the lock-up clutch is likewise gradually reduced, there appears another problem that acceleration behavior of the engine is unavoidably degraded.